Let's address the elephant in the room first and I quote the answer by zessx to the request mentioned in the first sentence:

There are major changes between CSS/CSS2 and CSS3. And sometimes you want to specify that your problem is related to these new features, as media queries, animations, etc. It's good to know that the question is not simply related to CSS.

There are differences between previous CSS-generations and CSS3. I think we can all agree on that. But I think what we should be asking is: Does it make sense to differentiate between the two in the context of Stack Overflow?

In my opinion: No. As seen in the facts-section most questions are already tagged with both tags. Let's discuss the remaining ~26,000 questions:

Any question falls in these two categories:

The question is about CSS in general

The question specifically about a CSS3-solution

In both scenarios css would apply as well as it does not exclude CSS3 (most of the ~500.000 questions only tagged with css have some CSS3-solution in their answers). You might argue that a question tagged with css3 is more specifically tagged which helps users to find them. This might even be true but time has shown that it is not working and most of the community (myself included) seems to have no idea when to use css3 (see When is the [css3] tag appropriate?) on this site.

Reading through the purpose of tags on this site I cannot think of an example where this change would make any difference so we might as well just clean up the confusion once and for all.

Back in the day it might have made a difference but just like with HTML and HTML5 time has eradicated this separation and today they are used synonymously. Since people are not able to correctly use these tags I see no point in distinguishing between them anymore.

Having many thousands of outdated answers that might have a better CSS3 solution is a problem. But at least they are tagged correctly.
– Hans PassantMay 29 '19 at 12:50

True, but you can usually tell that simply from the post timestamp. Typically, if it looks right, but it's several years old, I'll make sure it's not deprecated first
– Sterling ArcherMay 29 '19 at 18:37

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A fact you didn't put there: only 9.1% of [css] questions are also tagged with [css3], according to your numbers.
– Cássio RenanMay 29 '19 at 19:22

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BTW, some other languages also have version tags that are used in conjunction with the "generic" tag. [c++] and [c++11], [c++14], etc is an example of this. So is [python] and [python-3.x]. Not trying to prove any point here, just adding this information in the mix. I don't really know here I stand on this.
– Cássio RenanMay 29 '19 at 19:28

2 Answers
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What I admire about Stack Overflow is, like in any good community, decisions once taken can be contested at a later point of time, based on newer circumstances. The same goes with the decision to not merge CSS and the CSS-3 tags, which seems to have been largely overruled because of the changed times.

Given that lots of top users in the tag have already voiced their support for the synonym, and the overwhelming community support in favor of it, I've added css3 as a synonym for css. That is css (× 587950) &leftarrow; css3 (× 79851).

how tag score is handled in such situation? all the points of the synomyn will get transfered to the main tag?
– Temani AfifMay 31 '19 at 10:34

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@TemaniAfif, the tag score will get transferred to the main tag. It's almost like 1 day or something, and then the tag badges recalc will happen the next day (shouldn't affect you).
– Bhargav Rao♦May 31 '19 at 16:53

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@Kaiido, the tag scores work well with synonyms. The only problem would be search, for example see this search, where you get a completely wrong number of results. I don't know if it is a bug or not, but synonyms are never reflected in the searches.
– Bhargav Rao♦May 31 '19 at 16:55

Ah then how come I still have my old different scores in html and html5? Cache?
– KaiidoMay 31 '19 at 23:37

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Weird, I've usually seen it to be added @Kaiido. I don't think you can blame caching for some 10+ days... Perhaps I'm mistaken. Anyway, I'll ask around if any one knows and then get back. Thanks for informing me about this.
– Bhargav Rao♦Jun 1 '19 at 0:05

To confirm that we no more need css3(and probably we never need it) here is the relevant part from the specification describing CSS levels:

CSS Level 3

CSS Level 3 builds on CSS Level 2 module by module, using the CSS2.1 specification as its core. Each module adds functionality and/or replaces part of the CSS2.1 specification. The CSS Working Group intends that the new CSS modules will not contradict the CSS2.1 specification: only that they will add functionality and refine definitions. As each module is completed, it will be plugged in to the existing system of CSS2.1 plus previously-completed modules.

From this level on modules are levelled independently: for example Selectors Level 4 may well be completed before CSS Line Module Level 3. Modules with no CSS Level 2 equivalent start at Level 1; modules that update features that existed in CSS Level 2 start at Level 3.

Then

CSS Level 4 and beyond

There is no CSS Level 4. Independent modules can reach level 4 or beyond, but CSS the language no longer has levels. ("CSS Level 3" as a term is used only to differentiate it from the previous monolithic versions.)

Basically CSS will not grow beyond Level 3, and starting from that level we will now focus on each module independently. CSS3 is simply telling us that we will stop leveling the whole CSS, but will level only modules.

So CSS3 is simply CSS and if we need to be more specific we add a module related tag (flexbox, css-grid, css-calc, etc)

Worth to note that most of the browsers have full support for the CSS2.1 specification, which means that they do for CSS3, but the support for different modules may differ depending on the module level. Technically, we cannot say a browser doesn't support CSS3, but rather it doesn't support a specific module at a specific level.

Yes, the key point here is that browser and browser version is much more relevant information than whether you're doing something that is "CSS3" or not. For example, currently all major browsers support nth-child but only Safari supports the of S syntax that is in the Selectors-4 Working Draft. CSS support is a per-feature, per-browser thing; CSS "versions" are irrelevant.
– TylerHMay 29 '19 at 14:59